In a projection type display device, as one technique to improve the contrast ratio of a projected image on a screen, technology has been already proposed in that a transparent liquid having the same refractive index as that of a face glass of a CRT (cathode ray tube) and a lens is filled in a space between the CRT and a lens element to enlarge its reflected image, and light reflection in such a face glass or lens is eliminated. Such technology in the prior art will be described referring to FIG. 5 which shows a CRT 1, a fluorescent layer 2, a face glass 3 of the CRT 1, lens elements 4,5, a liquid container 6, a liquid sealing O-ring 7, a transparent liquid 8, and a screen 9.
In FIG. 5, the liquid container 6 having a circular or rectangular cylinder shape is installed between the face glass 3 of the CRT 1 and the lens element 5 having a collar on the outer circumference so that the face glass 3, the lens element 5 and the liquid container 6 constitute a space. This space is sealed by the O-ring 7, and the transparent liquid 8 is charged within the sealed space. In this case, the face glass 3 and the lens element 5 are nearly equal in refractive index, and the refractive index of the transparent liquid 8 is selected to be a value of about 1.4-1.6 which is close to, i.e., substantially equal to, the refractive index that of the face glass 3 and the lens element 5. According to such arrangement, light emitted from the fluorescent layer 2 of the CRT 1 passes through the face glass 3, the transparent liquid layer 8, the lens elements 5, 4 and arrives at the screen 9, and an image reflected to the fluorescent layer 2 is enlarged and then projected and displayed on the screen 9. Since the refractive index of the transparent liquid layer 8 is nearly equal to that of the face glass 3 or the lens element 5, the light emitted from the fluorescent layer 2 is prevented from being reflected on the exit side surface of the face glass 3 contacting the transparent liquid layer 8 (surface at the side of the screen 9) or the incidence side surface of the lens element 5 contacting the transparent liquid layer 8 (surface at the side of the CRT 1). Consequently, useless light on the screen 9 produced due to such reflection is reduced and the contrast ratio is improved.
However, the contrast ratio which can be attained by such prior art is restricted to about 100 to 1. Therefore, a problem exists in that a portion of the image which is originally black in the reproduced image on the screen 9 becomes gray, and decipherability of the black figure is deteriorated. Also when the reproduced image is a color image, a problem exists in that color purity is deteriorated. Such problems will be described referring to FIG. 5. In FIG. 5, regarding light emitted from a peripheral part P1 of the fluorescent layer 2 to which an image is reflected, some portion of this light is directed to the center of the lens element 5, but depending on the position of the peripheral part P1, since the lens element 5 is a curved surface and the lens element 5 and air are different from each other in refractive index, the light directed to the center is subjected to total reflection at the center of the exit side surface of the lens element 5 and returned to a peripheral part P2 of the fluorescent layer 2. If such a light exists, this light causes the peripheral part P2 to emit a light, which arrives at the screen 9 as above described. In such manner, if unrequired light (hereinafter referred to as "useless light") strikes the screen 9, the contrast ratio of the display image on the screen will be deteriorated.
A light emitted from a position P3, which is not at a peripheral part of the fluorescent layer 2, is not subjected to total reflection at the center of the lens element 5, but may be subjected to total reflection at a peripheral part of the lens element, and a light subjected to the total reflection at the peripheral part is further reflected at another part, for example, an inner wall of the liquid container 6, and returned to position P4 of the fluorescent layer 2 again. Also in this case, a light is emitted at the position P4, and the emitted light becomes useless light on the screen 9.
Further although not shown in FIG. 5, a light emitted from the fluorescent layer 2 may be diffused and reflected at the inner wall of the liquid container 6 and returned to the fluorescent layer 2 again, so that the contrast ratio may be deteriorated. Also, although not shown in FIG. 5, the lens element 4 is surrounded by a cylindrical mount separately so that the external light from the surrounding illumination source is prevented from entering from the face glass 3 into the CRT 1 to the minimum.